A man with blood group A , married to a woman blood group B , had a child with group AB . What does this mean in terms of genetics?

Correct option is A

In the ABO blood group system, the alleles for A and B are both dominant, while the O allele is recessive.

• ​ A person with blood group A can have the genotype AA or AO.
  
• A person with blood group B can have the genotype BB or BO.
  
•  A person with blood group AB must have the genotype AB, inheriting one A allele from one parent and one B allele from the other parent.

In this case:

• ​The man with blood group A (likely AO genotype) provides the A allele.
  
• The woman with blood group B (likely BO genotype) provides the B allele.
  
• Their child, with blood group AB, inherits both the A allele from the father and the B allele from the mother.
  
• This shows that both A and B alleles are dominant, which is why the child has the AB blood group.

Thus, the correct answer is (a) Gene for blood group A and gene for blood group B are both dominant.